1. Field of the Invention
The present invention relates to a semiconductor memory device, and more particularly to a semiconductor memory device having a redundancy rescue function.
2. Description of the Background Art
In recent years, various techniques for rescuing a semiconductor memory device having a faulty memory cell so as to improve a rate at which good quality is obtained have been suggested in accordance with a semiconductor memory device having an increased capacity. As one of the suggested techniques, a technique in which a spare memory cell is previously provided, and a faulty memory cell is replaced with the spare memory cell is suggested. The spare memory is also called a redundant memory, and a semiconductor memory device having the redundant memory is called a semiconductor memory device having a redundancy rescue function.
Whether or not the main memory cells other than the spare memory cells include a faulty memory cell is determined through testing using a tester. Whether or not a faulty memory cell is included depends on a voltage condition. Therefore, in the testing using a tester, whether or not the fault memory cell is included is determined by changing voltage conditions.
In the testing using a tester, when a voltage condition is changed, an influence of surge and the like may cause an erroneous measurement. Therefore, in general, power is turned off each time a voltage condition is changed. A result of testing under a stable voltage condition can be obtained when testing is performed by turning off power each time a voltage condition is changed. These conventional arts are disclosed in, for example, Japanese Laid-Open Patent Publication No. 2003-36696 and Japanese Laid-Open Patent Publication No. 11-16390.
In a case where the testing is performed by turning off power each time a voltage condition is changed, the result of the testing under the voltage condition which is obtained before power is turned off may be erased unless the result of the testing is stored in an external tester device.
In recent years, a self repair function called a BISR (Built-In Self Repair) is incorporated in a semiconductor memory device in many cases. The semiconductor memory device having the self repair function incorporated therein detects for a faulty memory cell therein without using an external tester device. However, when a conventional semiconductor memory device having the self repair function incorporated therein is powered off each time a voltage condition is changed, so as to perform the testing, the result of the testing which is obtained before power is turned off is erased because an external tester device is not used. Therefore, on the assumption that, in the conventional art, the semiconductor memory device having the self repair function incorporated therein is to perform the testing by turning off power each time a voltage condition is changed, it is impossible to determine, by using the results of the testings under all the voltage conditions, whether or not the rescue is to be performed.